The invention concerns a horn antenna of the type presented in the introduction to claim 1, for radiating or receiving polarized electromagnetic waves.
These horn antennas are especially used when there is a need for low cross-polarization and possible low side lobes across a large frequency area, for example, as a feeding element in reflector antennas or as individual antenna element in the micro or millimeter-wave areas.
Corrugated horn antennas, which are commonly used for the above purposes, are referred to in the following works: R. E. Lawrie et al. "Modifications of Horn Antennas for Low Sidelobe Levels," IEEE Trans.Antennas Propagat., vol. AP-14, September 1966, pp. 605-610; H. C. Minnett et al. "A Method of Synthesizing Radiation Patterns with Axial Symmetry", IEEE Trans.Antennas Propagat., vol. AP-14, September 1966, pp. 645-646. These horn antennas are, however, difficult to produce commercially, especially in the millimeter-wave area.
Other corrugated horn antennas are described in the following works. P. J. B. Clarricoats et al, "Theoretical Analysis of Cylindrical Hybrid Modes in a Corrugated Horn", Elektron. Lett., vol. 5, May 1, 1969, pp. 187-189; and P. J. B. Clarricoats, "Analysis of Spherical Hybrid Modes in a Corrugated Conical Horn," Elektron. Lett., vol 5, May 1, 1969, pp 189-190. In these corrugated horn antennas the horn wall is made anisotrope and reactive, and it complies with the balanced hybrid condition of the hybrid HE.sub.11 mode within the desired frequency band. Thus, the diagram of radiation in the E and H plans will become almost alike and give low cross-polarization.
Even though this type of antenna has in principle, satisfactory characteristics, it is burdened with disadvantages in regards to production.
The main object is, therefore, to create a horn antenna that has good electrical properties and is easy to manufacture. According to the invention, this can be achieved by developing the antenna in accordance with the characterizing part of claim 1.
Additional characteristics of the invention are given in the sub-claims.
It shall be pointed out that dielectric horn antennas are wellknown from, for example, P. J. B. Clarricoats and C. E. R. C. Salema, "Antennas Employing Conical Dielectric Horns," Proc.Inst.Elec.Eng., vol.120, July 1973, pp. 741-756; and U.S. Pat. Nos. 3,414,903, 3,430,244 and 3,611,391. These consist of a plastic conical waveguide with a low refractive index, excited at the apex from a little horn antenna. Even if such hybrid mode antennas have low cross-polarization, a problem is created when the junction between the excitation horn and the plastic cone emits unwanted radiation. Moreover, the radiation poperties are quickly reduced if rain or pollution falls on the plastic wall. This means that these antennas must be covered with a radome, which adds to the costs of the construction.
Another familiar horn antenna with low cross polarization is the bimode horn, described by P. O. Potter, "A New Horn Antenna with Improved Sidelobes and Equal Beams," Microwave J., vol. 11, June 1963, pp. 71-78. This has, true enough, a simple design, but with a narrow band width compared with the antennas described above.
The invention offers an advantageous alternative to wellknown hybrid and bimode horn antennas, and will, in many instances, be preferable.
The invention will be described in more detail below: